1. Field of the Invention
This invention relates to solenoid drive systems in general and more particularly, to those systems which include a solenoid drive system in which accurate control of the duration and force of the output of the solenoid, irrespective of voltage variations, is required. More specifically, this invention relates to the accurate control of a high speed impact solenoid driven printer to provide accuracy of control for print quality purposes.
2. Description of the Prior Art
Printers which use the so-called daisy wheel and high speed impact hammer principle are well known and are currently commercially available.
Accurate control of the printer is required to provide good print quality. Several techniques have been employed to analyze and thus control the force, flight time and duration of force of a print hammer. These variables have been applied in accordance with the print area to be printed, the number of forms to printed, etc. Following are some of the issued patents which have approached the hammer control problem.
Berry, U.S. Pat. No. 3,712,212, filed Nov. 12, 1971, issued Jan. 23, 1973 is illustrative of an impact printer in which the force for printing is varied in accordance with the surface area of the character being printed. In Berry a rotary print wheel or drum or an endless belt is used and one or more print hammers are cooperable with the member to print upon the print media. The electromagnetic field produced by a solenoid, as is typical, initiates the flight of the hammer against a document. While Berry addresses the problem of hammer impact based on surface area, no attempt is made in Berry to control the hammer force to compensate for voltage supply variations. Instead, the pulse applied to the solenoid coil of the print hammer is timed from the application of the pulse without any consideration given to any variation in the rise time of the pulse occasioned by fluctuations in power supply voltage or inductance variations.
U.S. Pat. No. 3,866,533 to Gilbert, et al, filed Dec. 26, 1972, issued Feb. 18, 1975 is another system for varying the impact of a print hammer. In this system the width of the pulse applied to the print hammer solenoid is varied in accordance with the thickness of forms on which printing is being performed. Secondarily, in this patent there is taught a smoothing technique for smoothing the input voltage to minimize the print hammer impact variations. In this patent, however, there is no technique taught of timing the print pulse from the time that a predetermined current level is reached in the solenoid coil to overcome voltage variation problems.
U.S. Pat. No. 4,030,591 to Martin, et al, filed Sept. 25, 1975, issued June 21, 1977 again shows a print hammer control circuit. In this system the print hammer is timed dependent on printing speed. Again, the hammer firing occurs based on the time that a pulse is received. The timing of the pulse is based on the printing speed. No attempt is made to exercise any control over the hammer after the gating pulse has been received. Likewise, the duration of the pulse is timed from the initiation of the current in the hammer coil and is not timed from the point that the current in the coil reaches a predetermined level. Thus, rise time variations can adversely affect the Martin system.